This invention relates to photoconductive organic polymeric materials and devices comprising a trans-rich polyphenylacetylene.
For a wide variety of electronic applications, it is highly desirable to utilize electrically conductive materials having photoconductive properties. A number of electrically conductive or semiconductive organic materials are known. In many instances, the conductivity of such material may be enhanced through the use of chemical doping techniques using various electron acceptor and/or electron donor dopants. However, although a number of electrically conductive organic materials are known, a substantially fewer number of such materials having photoconductive properties is known. Furthermore, the photoconductive properties of a material are not readily predictable from a knowledge of the electrical properties. Therefore, a continuing need exists for the discovery of materials having photoconductive properties for use in various photodetector and photoconductive devices, reprographic systems, light wave communication systems and the like. Devices utilizing photoconductive materials are well known in the literature and are described, for example, in Topics in Applied Physics; Optical and Infra-red Detectors, Ed. R. J. Keyes, Springer-Verlag, 1977.
One organic material known to exhibit electrical conductivity is polyphenylacetylene, a conjugated polymer that forms in both the cis- and trans- configurations and that may be doped with chemical dopants such as iodine or arsenic pentafluoride to improve the electrical conductivity thereof.
Polyphenylacetylene having molecular weights, for example, in excess of about 10,000 can be prepared by various methods, for example, by polymerization of phenylacetylene in the presence of a transition metal catalyst, such as ferric acetylacetonate-triethylaluminum catalyst. It has been found that the use of ferric acetylacetonate-triethylaluminum catalyst results in a polyphenylacetylene wherein the cis-configuration dominates. The preparation of polyphenylacetylene by polymerization using such catalysts, and discussions of some of the properties of the polymer, is disclosed in Sanford, T. J. et al; J. Polymer Science: Polymer Physics Edition, 18, 2277 (1980) and in Kang et al; J. Polymer Science, Polymer Letters Ed. 20, 143 (1982), wherein it is also disclosed that cis-polyphenylacetylene exhibits photoconductive properties.
It is also known that phenylacetylene may be polymerized in the presence of a tungsten catalyst such as tungsten carbonyl or tungsten hexachloride to form a trans-rich polyphenylacetylene. By the terms "trans-polyphenylacetylene" and trans-rich polyphenylacetylene", as used herein, is meant that the polyphenylacetylene referred to contains at least about 60 weight percent of the polyphenylacetylene in the trans-configuration. The preparation of trans-rich polyphenylacetylenes and the electrical conductivity thereof, in both the doped and undoped state, is disclosed, for example, in Kuwane et al, Polymer Journal, Vol. 12, No. 6 pp. 387-391 (1980). Thus, it is known in the literature that polyphenylacetylene is electrically conductive in both the cis- and trans-configurations. It has now been found, surprisingly, that trans-polyphenylacetylene exhibits distinctly advantageous photoconductive properties when compared with the cis-form of the same polymer.